1. Field of Invention
The present invention pertains to the field of cyclotrons. More particularly, this invention is a cyclotron construction including permanent magnets.
2. Description of the Related Art
In the field of nuclear medicine, it is well known that cyclotrons are used for producing radiopharmaceuticals for use in imaging. Conventional cyclotrons employ a concept called “sector focusing” to constrain the vertical dimension of the accelerated particle beam within the poles of the cyclotron magnet. The magnet poles contain at least three wedge-shaped sectors, commonly known as “hills”, where the magnetic flux is mostly concentrated. The hills are separated by regions, commonly referred to as “valleys”, where the magnet gap is wider. As a consequence of the wider gap the flux density, or field strength, in the valleys is reduced compared to that in the hills.
Vertical focusing of the beam is enhanced by a large ratio of hill field to valley field; the higher the ratio, the stronger are the forces tending to confine the beam close to the median plane. In principle, a tighter confinement, in turn, reduces the required magnet gap without danger of the beam striking the pole faces in the magnet. For a given amount of flux in the gap, a magnet with a small gap requires less electrical power for excitation than does a magnet with a large gap.
In the limiting case of the “separated sector cyclotron” each hill sector is a complete, separate, stand-alone magnet with its own gap, poles, return/support yoke, and common excitation coil. In this implementation the valleys are merely large void spaces containing no magnet steel. Essentially all the magnetic flux is concentrated in the hills and almost none is in the valleys. In addition to providing tight vertical focusing, the separated-sector configuration allows convenient placement of accelerating electrodes and other apparatus in the large void spaces comprising the valleys.
More recently, superconducting magnet technology has been applied to cyclotrons. In superconducting cyclotron designs, the valleys are also large void spaces in which accelerating electrodes and other apparatus may be conveniently emplaced. The magnet excitation for a superconducting cyclotron is usually provided by a single pair of superconducting magnet coils which encircle the hills and valleys. A common return/support yoke surrounds the excitation coil and magnet poles.
To this extent, currently conventional cyclotrons consist of a plurality of hollow, semicircular metal electrodes 12P, as illustrated in FIG. 1. These electrodes are commonly referred to as “dees” because of their shape. For simplicity, illustrated are two dees 12P. However, there are typically four or more dees 12P used. As will be discussed below, ions are accelerated in a substantially circular, outwardly spiraling path. In devices using fewer dees 12P, either more turns are required, or a higher acceleration voltage is required, or both, in order to energize the ions to the desired level. The dees 12P are positioned in the valley of the large electromagnet (not shown). Near the center of the dees 12P is an ion source 34P used for generating charged particles. The ion source 34P is typically an electrical arc device 50 in a gas.
During operation, ions are continuously generated by the ion source 34P. A filament located in the ion source assembly creates both negative and positive ions through the addition of electrons or the subtraction of electrons. As the negative ions enter the vacuum tank 28P, they gain energy due to a high-frequency alternating electric field induced on the dees 12P. As the negative ions flow from the ion source 34P, they are exposed to this electric field as well as a strong magnetic field generated by two magnet poles, one above and one below the vacuum tank 28P. Because these are charged particles in a magnetic field, the negative ions move in a circular path.
When the negative ions reach the edge of the dee 12P and enter the gap, the RF oscillator changes the polarities on the dees 12P. The negative ions are repelled as they exit the previously positive but now negatively charged dee 12P. Each time the particles cross the gap they gain energy, so the orbital radius continuously increases and the particles follow an outwardly spiraling path. The particles are pushed from the first dee 12P and drift along a circular path until they are attracted or pulled by the second dee 12P which has become positively charged. The result is a stream of negative ions which are accelerated in a circular path spiraling outward.
Cyclotrons that are typical of the art are those devices disclosed in the following U.S. patents:
U.S. Pat. No.Inventor(s)Issue Date1,948,384E. O. LawrenceFeb. 20, 19344,206,383V. G. Anicich et al.Jun. 3, 19804,639,348W. S. JarnaginJan. 27, 19875,463,291L. Carroll et al.Oct. 31, 19955,818,170T. Kikunaga et al.Oct. 6, 19986,060,833J. E. VelazcoMay 9, 20006,163,006F. C. Doughty et al.Dec. 19, 20006,396,024F. C. Doughty et al.May 28, 20026,523,338G. Kornfeld et al.Feb. 25, 20032004/0046116J. B. Schroeder et al.Mar. 11, 20042006/0049902L. KaufmanMar. 9, 2006
Of these patents, Lawrence, in his '384 patent, discloses a method and apparatus for the acceleration of ions. The Lawrence patent is based primarily upon the cumulative action of a succession of accelerating impulses, each requiring only a moderate voltage, but eventually resulting in an ion speed corresponding to a much higher voltage. According to Lawrence, this is accomplished by causing ions or electrically charged particles to pass repeatedly through accelerating electric fields in such a manner that the motion of the ion or charged particle is in resonance or synchronism with oscillations in the electric accelerating field or fields.
Anicich et al., in their '383 patent, disclose a miniaturized ion source device in an air gap of a small permanent magnet with a substantially uniform field in the air gap of about 0.5 inch. The device and permanent magnet are placed in an enclosure which is maintained at a high vacuum (typically 10−7 torr) into which a sample gas can be introduced. The ion-beam end of the device is placed very close to an aperture through which an ion beam can exit into apparatus for an experiment.
Jarnagin, in his '348 patent, discloses a re-circulating plasma fusion system. The '348 patent claims to include a plurality of recyclotrons, each comprising cyclotron means for receiving and accelerating charged particles in spiral and work conservative pathways, and output means for forming a beam from particles received. The cyclotron means used by Jarnagin includes a channel shaped electromagnet having a pair of indented polefaces oriented along an input axis and defining an input magnetic well. The cyclotron further includes a pair of elongated linear electrodes centered along the input magnetic well arranged generally parallel to the input axis and having a gap therebetween. A tuned oscillator means is connected to the electrodes for applying an oscillating electric potential thereto. The output means includes an inverter means including an electromagnet having a polarity opposite that of the channel shaped electromagnet oriented contigously therealong for extracting fully accelerated particles from the cyclotron means. A reinverter means includes an electromagnet having a polarity the same as that of the channel shaped electromagnet for correcting the flight path of the extracted particles, the inverter means and the reinverter means defining an output axis, along which the output means directs the beam. The recyclotrons are arranged so that particles of the output beam are received by the input magnetic well of an opposing similar recyclotron.
Carroll, et al., in their '291 patent, disclose a cyclotron and associated magnet coil and coil fabricating process. The cyclotron includes a return yoke defining a cavity therein. A plurality of wedge-shaped regions called “hills” are disposed in the return yoke, and voids called “valleys” are defined between the hills. A single, substantially circular magnet coil surrounds and axially spans the hills and the valleys.
In the '170 patent, Kikunaga et al., disclose a gyrotron system including an electron gun that produces an electron beam. A magnetic field generating unit comprises a permanent magnet and two electromagnets, and is capable of generating an axial magnetic field that drives electrons emitted from the electron gun for revolving motion. A cavity resonator causes cyclotron resonance maser interaction between the revolving electrons and a high-frequency electromagnetic field resonating in a natural mode. A collector collects the electron beam that has traveled through the cavity resonator. An output window is provided, through which a high-frequency wave produced by the cyclotron resonance maser interaction propagates.
Velazco, in the '833 patent, discloses an electron beam accelerator utilizing a single microwave resonator holding a transverse-magnetic circularly polarized electromagnetic mode and a charged-particle beam immersed in an axial focusing magnetic field.
In their '006 patent, Doughty et al., disclose a plasma-producing device wherein an optimized magnet field for electron cyclotron resonance plasma generation is provided by a shaped pole piece.
In their '024 patent, Doughty et al., disclose a method and apparatus for integrating multipolar confinement with permanent magnetic electron cyclotron resonance plasma sources to produce highly uniform plasma processing for use in semiconductor fabrication and related fields. The plasma processing apparatus includes a vacuum chamber, a workpiece stage within the chamber, a permanent magnet electron cyclotron resonance plasma source directed at said chamber, and a system of permanent magnets for plasma confinement about the periphery of the chamber.
Kornfeld et al., in the '338 patent, disclose a plasma accelerator arrangement in particular for use as an ion thruster in a spacecraft. A structure is proposed in connection with which an accelerated electron beam is admitted into an ionization chamber with fuel gas, and is guided through the ionization chamber in the form of a focused beam against an electric deceleration field, said electric deceleration field acting at the same time as an acceleration field for the fuel ions produced by ionization.
In Published Application No. 2004/0046116, Schroeder et al., disclose a negative ion source placed inside a negatively-charged high voltage terminal for emitting a beam which is accelerated to moderate energy and filtered by a momentum analyzer to remove unwanted ions. Reference ions such as carbon-12 are deflected and measured in an off-axis Faraday cup. Ions of interest, such as carbon ions of mass 14, are accelerated through 300 kV to ground potential and passed through a gas stripper where the ions undergo charge exchange and molecular destruction. The desired isotope, carbon-14 along with fragments of the interfering molecular ions, emerges from the stripper into a momentum analyzer which removes undesirable isotope ions. The ions are further filtered by passing through an electrostatic spherical analyzer to remove ions which have undergone charge exchange. The ions remaining after the spherical analyzer are transmitted to a detector and counted.
In Published Application No. 2006/0049902, Kaufman defines a plurality of permanent magnets to enhance radiation dose delivery of a high energy particle beam. The direction of the magnetic field from the permanent magnets may be changed by moving the permanent magnets.